Sound recording and reproducing apparatus



Feb. 6, 1953 A. A. SHOUP ET AL SOUND RECORDING AND REPRODUCING APPARATUS 4 Sheets-Sheet 1 Filed Aug. 27, 1946 mvmvwwmvmv flllllllllllli INVENTORS ALLEN A. SHOUF RUDOLPH AB/ERWIRTH ATTY.

Feb. 6, 1951 A. A. SHOUP ET AL 2,540,299

SOUND RECORDING AND REPRODUCING APPARATUS Filed Aug. 27, 1946 4 Sheets-Sheet 2 ALLEN A. SHOUF RUDOLPH A. 51512 W/RTH EARLL OLSON ATTY Feb. 6, 1951 A. A. SHOUP ETAL souun RECORDING AND REPRODUCING APPARATUS 4 Sheets-Sheet 3 Filed Aug. 2'7, 1946 INVENTORS ALLEN A. SHOUF RUDOLPH A. B/ERWIRTH EARL L. 0LSON MVZ A. A. SHOUP ETAL SOUND RECORDING AND REPRODUCING APPARATUS Feb. 6, 1951 v 4 Sheets-Sheet 4 INVENTORS ALLEN A. SHOUP m m M E B A w L 0 w R N O O L L. m E

ATTY

Filed Aug. 27, 1946 Patented Feb. 6, 1951 SOUND RECORDING A ND REPRODUGING: APPARATUS Allen A. Shoup, Chicago, Rudolph Bierwirth- Mount Prospect, and'Earl L.- Olson, Qlricagor Ill., assignors to The George W.-Borg"-Corpora tion, Chicago, Ill., a corporation of Delaware Application August 27, 1946, Serial No. 6931190" 20 Claims; 1 The present invention relates in general to sound recording and reproducing apparatus, and more in particular to apparatus of this character in which sound is recorded electro-mag-' netically on a para-magnetic. wire or similar medium.

The particular sound recording and repro- V ducing apparatus which is described herein has been designed specifically for use as an ofiice dictating and transcribing machine, although certain features of. the apparatus will no doubt prove to be useful in other fields- The. object of the. invention is the production of a novel and highly efiicient' audio. frequency amplifier for an apparatusoi the foregoing character- A special object the. provision of an audio frequency amplifier having. a new and improved automatic volume control- Fig. 1 is. a frontview of the mechanical unit employed for handling the wire;

.. Fig.2 is a top View of the-mechanical unit;

Fig.3 is arear View of the same;

Fig. 4' is a diagrammatic circuit drawing of a complete ofiice dictating and transcribing machine embodying the. invention; and

Fig. 5 is aview of the hand microphone and switching unit employed for dictation.

The-mechanical unit for handling the wire will be. described first, referencev being, had. to Figs. 1, 2. and 3.

The various parts are mounted on a frame which includes two. vertically disposed frame; plates and 2|. These plates are held together 23,.at-the top, and by the cross members 24 and 25, at-the bottom. The-frame plates ma be fastenedto the cross. members by screws, as shown in the drawings.

The reference character. 26., Fig. 2., indicates. a motorwhich is. mounted on the frame in any suitablemanner. A indicated inFig. 2, the motor is attached toframe plate 2| by means of two of the. through bolts which hold: the motor frame together. These two bolts aresomewhat longer than usual and are threaded into tapped holes in the frame plate 2|. The: motor 26- may be an A. C. capacitor type motor and has two output shafts running at difierent speeds.

Shaft 2"listhe armature shaft'and preferably has aspeed of 3600 R. P. M. Shaft'ZB is connected to shaft" 21 by speed reducing gears and preferably has a speed of 240 R. P. M.

The wire 30' is of para-magnetic material and is carried on spools 3| and" 32; Spool3l may'for ever, identicai and interchangeable. The wire 2 30 is attached to spool 32 in known manner by" means of a leader 33, which is wrapped several times around the spool: This leader is made of thin flexible plastic material, has serrated or scalloped edges; and" is slightly wider than the spool. The other end of' the wire has a similar leader bymeans'of' which it'isJ attached to spool 3|. These lead'er'sarelong enough so that wheni the wire is wound entirely oiffrom one spool on.

to the other; as from. spool 32 to spool 31, the leader at the end'o'f the wire; in this case leader 33, will be part1; wound on spool 31 before it is entirely'unwound from spool 32', thus preventing 'g f the. outer turns of wire on the:

formerspool.

The spool 32is rem'ovablymounted on a flanged hub 35, which is fi'xedon the shaft 36'. The spool? is retained on the hub. by a spring pressed-ball 31, mounted in a' radial hole in. the hub; which 43. and 44, secured. to the. shaft-36 by means of set screws, prevent endwise movement thereof.

The shaft 40 is rotatably mounted on" the frame;

in the same way, by meansof the; flanged; bear- 1 ing bushings 45and46. in spaced relation by the cross members 22 and During the. operation of" the. machine f01"1'8=.-

cording, and. also for" reproducing, the take; I

spool 32 is: driven by'themQtorrZS. at slow speeds in a clockwise direction as. seen in Fig. 1. This: may'bereferred t'o as th 'slow' forward drive; and is effected by means of; a'- roller-41 on the-motor shaft 28; a. roller 49 ontheshaft 3'6', and a rubber tired idler roller 48-. The latter rolleris mounted on the vertically slidable member 50' by means of a stud or" shoulder screw 5|. The mem ber 50 is supported against the frame plate 20 by means of the washer'53and the shoulder screw will he noted also that-the mem-ber'fifl is'-pi-vota1ly connected to the bell, crank lever 54% Them'ember'lrfl'is normall'yheld in the position in which it is shown in? Fig: l, with roller rout: of' engagement with: rollers 4-1 and. 49*, by means -of the spring 56. which urges. the link" 55' to the left and holds the lower arm of lever 54 against; the stop pin 51} Roller 48 is hrought into. en.-'

gagement' with rollers 4T and! by means ofthe solenoidh5'81Fig. 2', which mounted on the back of frame plate 20 by means of the two screws 59, Fig. 1. The core 60 of the solenoid carries an L' shaped bracket 6| which isconnected to the crank 93 by means of the shoulder screw 62. This screw passes through a slot in the crank 63 and is threaded into the bracket 6 I. The crank 63 is fixed on the shaft 64, which has bearings in frame plates 20 and 2I. Another crank 65 connects the shaft 64 with the link 55. When solenoid 58 is energized, the core 60 and bracket GI move to the right as seen in Fig. 1 and the shaft 64 is rotated by means of crank 63. The shaft 64 moves the link 55 to the right by means of crank 65, and the link 55, by means of bell crank lever 54, pulls the member 50 downward and thus operatively engages roller 48 with the rollers 41 and 49.

The reference character 68 indicates a normally closed switch of well known construction. This switch is mounted on the frame plate 20 and is held open during slow forward drive by the bell crank lever 54.

The reference character 61 indicates a centrifugal switch. This switch is associated with the shaft 40 and may be of any suitable and known type. As shown in Fig. 2, it comprises two semi-cylindrical members 68 and 99 having lugs I and H by means of which they are mounted on the disc I2. The latter is made of insulating material and is secured to the frame plate 20. The lugs I0 and II are the terminals of the switch. The casing 6'I is mounted on shaft 40 and contains three pivoted switch members, electrically connected but insulated from shaft 40, which are held in contact with the semi-cylindrical members 58 and B9 bymeans of springs, thereby normally maintaining the switch closed. The, springs are adjusted so that the switch opens whenthe shaft 49 rotates at a speed slightly higher than the slow forward speed.

The slow forward drive which has been described is used during recording and reproducing. The machine also has a fast forward drive which makes it possible to skip a part of the record and bring a desired part thereof quickly to recording position. The motor shaft 2'! is employed on the fast forward drive, as well as on the rewind drive.

The motor shaft 21 carries a pulley I5, Figs. 2 and 3, which may be coupled by means of belts I8 and I9 to the pulleys I5 and TI carried on shafts 39 and 40, respectively. The belts are normally loose, so that pulley I5 can rotate freely without applying any .driving torque to either pulley IE or 11. The guide pulleys 80 and BI are mounted on the adjustable brackets 82 and 83 and afford means for adjusting the belts I8 and I9, respectively.

The belts are rendered selectively operative for driving purposes by means of the bell crank levers 84 and 85 carrying the idier rollers 86 and 8'! which are adapted to engage the belts I8 and I9, respectively. The levers 84 and 85 are selectively operated by means of the fast forward and rewind solenoids 88 and 89, respectively. These solenoids are mounted on the inside of frame plate 2I, as shown in Fig. 2, by means of screws such as the screws 59, Fig. l, which secure the solenoid 58 to frame plate 20. The cores 90 and 9| of the solenoids are connected together by the bar 92, shown clearly in Figs. 2 and 3. This bar is provided with the two studs 93 and 94, which 7 are connected with the levers 84 and 85 by springs 95 and 96, respectively. Each spring includes a straight end section which passes through a diametral hole in the associated stud. In the case of spring 95 this end section is indicated at 91.

Just beyond the stud 93 there is a short piece of perforated rod 98 which is secured to the end section 91 of the spring by a set screw. With this construction the stud 93 pulls on the spring when it moves to the left but is free to slide along I 91 when it moves to the right. The spring 96 is attached to stud 94 in the same way.

When the fast forward solenoid 98 is energized, the bar 92 is moved to the left, as seen in Fig. 3. The stud 93, being fixed. to bar 92, accordingly moves to the left also and operates the bell crank lever 84 by means of spring 95. The roller 86 is thus pressed against the belt I8, tightening the belt, and causing the pulley I5 to drive the pulley IS. The direction of rotation is shown by the arrows on these pulleys and is such that the spool 32, Fig. l, is driven in the same direction as on the slow forward drive.

'When the rewind solenoid 89 is energized, the bar 92 is moved to the right and operates the bell crank lever by means of stud 94 and spring 96. This operation tightens the belt I9 and causes the pulley 15 to drive the pulley TI. The direction of rotation, indicated by the arrows, is such that the spool 3 I Fig. l, is driven in a clockwise direction to rewind the wire 30 from spool 32 to spool 3|.

Following the deenergization of each solenoid the bar 92 is restored substantially to the position in which it is shown in Fig. 3. Restoration is accomplished by the belts l8 and I9, reacting againstthe rollers 86 and 81.

The pulleys l6 and I1 are provided with brakes, together with means for selectively applying the brakes at times when overrun is apt to occur. This mechanism will now be described.

The brake for pulley I I comprises the lever I00.

pivoted at I02, the brake band IOI attached to lever I80, and the light spring I05, which tends to rotate the lever on its pivot in a clockwise direction. The brake band IOI may be made of untreated woven belting, the same as used for the belts i8 and I9, but rubber impregnated belting is preferred because of the greater coefficient of friction between the brake band and the face of the pulley. One end of the brake band IIII is more or less permanently attached to the lever I00 near the roller I03, while the other end is clamped against the lower side of the lever near the pivot I02 by means of the U shaped piece I05 and the hexagonal headed screw I01. This arrangement makes it possible to adjust the length of the brake band. The spring I05 normally holds the brake band taut so that some friction is developed between the brake band and the face of the pulley when the pulley is rotating which retards the rotation. When the pulley is rotating in a clockwise direction the pull on the brake band aids the spring I05, that is, it tends to tighten the brake band, and a substantial braking action results. When the pulley is rotating in the opposite direction, however, the pull.

on the brake band tends to loosen it and the braking action is slight.

The brake for pulley 16 comprises the pivoted lever I08, the brake band I09 and the light spring H0. The construction and operation is the same as described in the case of the brake for pulley TI.

The brakes are self actuating and are positively released by means of cam mechanism which will now be described.

There are four cams, two of which, cams H2 and H3, cooperate with the roller III on lever I08, while the other two, cams H4 and H5, cooperate with the roller I04 on lever I00. Cams 5. H2. and I I4 are carried: on the bar 92, already described,.while the cams H3 and lii5' are carried. on. the auxiliary bar I I6- The bar I-I'Ii eX-- tends parallel to bar. 92 and: is supported on. the

two shoulder screws II! and :8 which pass 7 through slots in bars I I t and 92- and. are threaded into the frame plate. 2|... Washers.- on. the shoulder screws are interposed between the two bars and between bar 92 and the frame plate 2| to hold the parts in spaced relation. The baa-I18 has two positions, to which it. is. moved. by the studs 93 and 94 on bar 92. These studs pass through slots in bar H6 as shown. In order to retain bar [It in one or the" other of its positions when not impelled to move by: one of the studs a. spring detent H9 is provided. This detentcooperates with. two notches in the lovver'edge of bar H6. I

In addition to the cam mechanism just described there is a lever I-20 on shaft 64 which cooperates with the roller I03 on lever I and. with the bar H6, as will be explained shortly.

I When the bar 92 is in normalposition, with, neither the rewind solenoid 88 nor the fastforward olenoid 88 energized, the cams I I2 and H4- are both inoperative. and the brakes are both on so far as the bar 92 is concerned. The bar I16, however, always maintains one or the other of the brakes released, the brake which is released depending on the position. of the bar. hand position, the position in which bar. H6 is shown in Fig. 3, the brake for pulley i6 is released while the brake for pulley I1 is on. The release or the. brake for pulley- I6 is accomplished by the cam 3 which is in engagement with roller III. and has rotated the lever I08 on its pivot. sufiiciently toloosen the brake: band I09.

It will be understood that the bar H6 is in. its left hand. position because of the previous energization of the slow forward. solenoid or the fast.- forward. solenoid, either of which is opera.-

tive. to move bar IIB from its right hand position.

to its left hand position. The rewind olenoid, when. energized, moves the bar to its right hand position. It will be convenient now to explain the brake control operations. which. take place upon the energization and deenergization of. these solenoids.

When the rewind solenoid 89 is energized, the. bar 92 is moved to the right,. the belt 19 is tightened by bell crank lever 85 to couple the pulley 11 to pulley I5, and the supply spool 3| is driven in the proper direction to rewind the wire 30 from spool 32 to spool 3I'. The movement of bar 92 to the right brings. about the movement of the bar II6 from its left hand position to its. right hand position, by the engagement of stud 84% with the right hand end of the slot in bar H6. The movement of bar I I6 to its right hand position causes cam H to engage roller I04, rotate lever I00 on its pivot, and release the brake on pulley TI. The movement of bar I It to its right hand position also causes the cam II3 to disengage roller I II but the brake on pulley 1-8 is not applied because it is maintained released by cam II 2- on bar 92, now moved to the position form-- erly occupied by cam II3'.

During the rewinding operation, therefore, both brakes are released and the spools are able to rotate freely, imposing a minimum load on the motor. When the solenoid 89 is deenergized tostop the rewinding operation, the bar 92 restores to normal position, thereby uncoupling pulley T! from pulley I5. The restoration of bar 92, by movingcam H2 out of engagement with roller It I", applies the brakeon pulley T6;

In its left.

The recording head is supported by a horiof a backlash due to overrunning of spool 32-.

ltimay now bev assumed. thatthe: slow forward solenoid 58. is energized, thereby coupling. the shaft36 on whichthe take up spool 32 is.mount-. ed to the slow speed motor shaft 28.. The spool 32 is now driven in. the. proper direction for. recording. or reproducing. The above; referred to coupling operation involves the rotation of shaftv 64 ina. clockwise direction, as seen in Fig. 3-,, causing. the lever I20 to move the bar I I6-to its left hand position. The cam I I5 is thus moved out of engagement with the roller I04 but the. brake. on pulley I'I is maintained released by the engagement. of lever I20 with the roller I03. The brake on pulley I6 having been released by cam II'3", both brakes are now in released. condition, as-was the. case during the rewinding. operation.

When the solenoid 58' is deenergized to stop the slow forward drive, bar II'6 remains in its'left hand position and cam I'I'3 holds the brake on. pulley T6 in released condition. The restoration: of lever I20, however, causes the brake to be applied on pulley IT, which brings the supply spool. 3 I to a stop. The take up spool 32 is stopped also, by the pull'on. the wire 30, and its inertia keeps 1 the wire taut, as in the previous case.

The brake operations responsive to the energization of the solenoid 88 for fast forward drive will readily be understood, inview of the preceding explanation. The' brake on pulley I1 is released by the" cam H4, and the bar I'IB, if not. already in its left hand position, is moved to that position by stud 93 to cause cam M3 to release the brake on pulleyIG. When the solenoid 88 is deenergized the brake is applied on pulley IT while the brake on pulley 16 remains released. The conditions are the same therefore as they are: upon the cessation of the slow forward drive and the spools are brought to rest in the same Way.

Summing up on the operation of the brakes,

when either" forward drive or the rewind drive is stopped, the spool to which power was applied is left to run free, except for the wire 30,

while the brake is applied on the pulley which is associated with the other spool. The towed spool is always positively brought to rest, therefore, while the previously d'ri'ven spool is stopped by the wire, which maintains the wire tight between the spools.

The recording head is indicated at I25 and may be of any known and suitable construction. The recording head developed by the Armour Research Foundation has been used with excellent results. This recording head comprises a core structure having two small air gaps, an erase coil associated with one air gap,'and a voice coil associated with the other air gap..

There is also a small biasing coil connected in series with the erase coil and associated with the voice coil air gap. The core structure with its windings is enclosed in a suitable casing as shown, carrying the pin terminals such as I26, two for the erase and biasing coils and two for the voice coil.

zontal angle member I3u which is in turn supported on the cross member 23 of the frame by means of the two flexible spring strips I32 and I33. The angle member I30 has an integrally formed depending and generally U shaped extension I3I on which the recording head I25 is mounted by means of two screws I34 and I35. The core I21 of a neutralizing coil I28 is supported on the front of the recording head by screw I35.

The level wind mechanism includes a shaft I36 which has bearings in the cross members 22 and 23 of the frame and is driven from shaft 36 by the worm I31 and worm gear I38. A heart shaped cam I39 is mounted on shaft I36. A stud I4I, rigidly secured to the angle member I30, has a roller I40 which cooperates with the cam I39. 'In this connection it will be understood that the flexible spring strips I32 and I33 are tensioned to maintain the roller I43 against the cam I36 so that the rotation of the cam is effective to cause a reciprocating motion of the recording head which traverses the wire 36 back and forth on the spools during winding in either direction.

An indicator may be provided comprising the drum 225, mounted on the wheel 226. The wheel 226 has a hub 221 and a short stub shaft 223, by means of which it is rotatably mounted on an L shaped bracket 228. The bracket is attached to the cross member 22 of the frame. The lower edge of the drum 225 has gear teeth formed therein which cooperate with the ,worm 238 on shaft I36, so that shaft I36 drives the drum.

The upper part of the drum 225 carries a scale which cooperates with the index or pointer 23I. The scale is calibrated in accordance with the amount of wire carried on a full spool. Assuming that the spool 3I carries sufilcient wire for one hours continuous dictation, with enough wire to provide for starting the wire on to spool 32 and to enable the full hours dictation to be completed without pulling the wire entirely 01f spool 32, the scale will have sixty divisions corresponding to the sixty minutes in the hour. The gear ratio between the slow speed motor shaft and the drum 225 issuch that the drum makes one complete rotation in an hour, if continuously driven. The scale is set to zero when starting off with a fresh spool of wire and as the wire is transferred to spool 32, or rewound temporarily on spool 3I, the drum is correspondingly rotated so that the scale always indicates the amount of wire in minutes of dictating time which has been transferred to spool 32.

It will be noted that the drum can be set to zero by merely raising it up far enough so that 'the gear teeth on the lower edge of the drum are disengaged from the worm 230 and then rotating the drum until the zero point on the scale is opposite the index 23I. The scale having been adjusted in this manner, the drum is lowered to normal position to bring the gear teeth into engagement with the worm again.

The frame which has been described, carrying the mechanical parts of the machine, may be mounted on a suitable base or chassis I42, partly shown in Figs. 1 and 3. This base extends beyond the frame at both ends and affords means for mounting various items of electrical equipment such as tubes, transformers, switches, etc. These items will be described with reference to the circuit drawing, Fig. 4. It will be convenient 8. at this time, however, to point out the switching keys A to E, inclusive, and R, which appear on the front of the base, where they can be reached easily for manual operation. Also mounted on the front of the base are the volume control switch I43, the main switch I44, and the conference switch I45. On the back side of the base, Fig. 3, are the jacks I46 and I 41, which may be ordinary telephone type jacks, and a suitable connector comprising the multiple pin plug I49 and the socket I48.

The hand microphone assembly I50 is shown in Fig. 5, and includes the microphone I52 and the switches I53 and I54 by means of which the apparatus is controlled when used as a dictating machine. The microphone I52 is mounted on one end of the casing I5I and is of the dynamic type, comprising a permanent magnet and a moving coil connected to the diaphragm. It is accordingly adapted to function either as a transmitter or a receiver. The Switches I53 and I54 are self-restoring and are mounted insidethe casing I5l. Switch I53 is operated by the push button I56, while switch I54 is operated by depressing the leaf spring I55. A multiple-conductor cord connects the assembly I50 with the plug I49. Referring now to Fig. 4, the hand microphone I52 is shown at the left, with the switches I53 and I54, and the connections to the plug I46 which is represented by a vertical row of eleven terminals. The socket I48 is represented by a similar vertical row of terminals,

Just above the plug I49 is shown a similar plug I66 which is adapted to be substituted for plug I49 when the machine is used by a transcribing operator. The plug I serves to connect the head phone I51 and the switches I58 and I59 to the machine by way of the socket I48. The switches I58 and I59 are mounted in a suitable casing (not shown) which rests on the floor where the switches can be actuated by the foot of the operator.

In the upper left hand corner the jacks I46 and I41 are shown, with their circuit connections.

' The key set, comprising the keys A to E, in elusive, and R, is a known type of key set in which the keys, except key R, are of the locking" tails reference may be made to Patent No. 2,-.

196,433, granted April 9, 1940. t

, There is a chain circuit comprising contacts E5, A5, etc., associated with the keys, which is included in the energizing circuits for the sole noids 58, 88, and 89 and makes it impossible to energize two of these solenoids at once by a false operation of the keys. Other contacts on the keys are shown separate therefrom according to known practice in order to simplify the circuit drawing.

Relay X is a switching relay, the function of which will be explained presently. This-relay also has contacts which are shown at different points in the drawing convenient to the circuits which they control.

The relay. I6I operates a ratchet mechanism} including the ratchet wheel I62, which is stepped ahead one tooth at each energization of the relay. The cam I63 rotates with the ratchet wheel and alternately closes and opens the contact I64,

The reference characters I65, I66, and I6! indicate, respectively, the voice coil, the biasing.

coil, --and the erase coil. These coils are in the recording head, as previously mentioned. The neutralizing coil I28 is connected in series with and in opposition to the voice coil. This neutralizing coil is employed because of the close proximity of the recording head to the motor 26, where it is affected by leakage flux from the motor field. Coil I128 is located as nearly as possible in the same position relative to the motor as the voice coil I65, and the voltages developed in the two coils by the leakage flux cancel each other. By properly shielding the recording head or by spacing it farther away from the motor, the coil I28 can be dispensed with.

The amplifier will now be jbriefiy described. It is a combination two and three stage amplifier.: All three stages are used during the reproduction or transcribing of a record, but during recording, or dictation, only two stages are used, the third stage being converted to an oscillator for supplying high frequency current to the erase and biasing coils.

The reference character I indicates the input transformer, having the primary winding HI and the secondary winding I12. The secondary winding is shunted by the load resistor I13 and the low capacity bypass condenser I14, and is included in the grid circuit of the tube I15. The tube I1'5 may be 'a type 12SL'7 triode and isith'efirst stage of the amplifier. The usual grid biasing resistor .is indicated at I16, shunted by the condenser I11. The anode circuit includes the resistor I11. The tube I89 maybe atype 128B"? diode-pentode and constitutes the second amplifier stage. The control grid of this tube is coupled :to the anode circuit of tube I15'by means of :the condenser I18 and the resistor I19. Condense I84 is a low capacity bypass condenser, similar to condenser I 14. The suppressor grid is connected tothe cathode while the screen grid is connected to a source of positive potential as shown. The anode circuit of the tube includes the primary winding I82 of the output transformer I8 I. 1' The tube I85 may be another type 1.2SL'1 triode and functions as an automatic volumecon'trol amplifier. 'Thegridof tube N35 i coupled to the anode circuit of tube I15 through the condenser I86 and resistor I81, in parallel with "the control grid of tube I89. The resistor 181 should have a high resistance, preferably about '1 megohm, and is provided in order to limit the flow of grid current in the tube. The anode circuit of tube I85 includes the primary winding I89 of the transformer I88, said winding being shunted by the load resistor I91 The secondary winding I90 of this transformer supplies power to a direct current circuit which includes the resistor 192 and I93 and the diode :elements of tube I80. From this circuit negative potentials for automatic'volume control .are obtained, .as will be explained more fully hereinafter. The condenser 194 is a filter condenser, connected across the resistors I92 .and I93, and preferably should have a'value of about 1 microfa'rad.

The tube 200 may be a type 35L6 pentode and is'adapted :to -funct-ionceither as the third stage of the amplifier or as an oscillator. The tube has two grid circuits, an anode circuit, and two output circuits inductivelycoupled to the anode circuit. The anode circuit includes the primary winding 292 of the oscillator transformer 20! and the primary winding 20.! of the amplifier output transformerZfiS. The winding 12.02;.issh1111ted by theiteele leeheen rgzta w i menin itis. 2.011.

and :grid circuits, respectively, -of thetube.

is shunted by the high frequency bypass condenser 2I3. The secondary winding 208 of transformer 206 is in the amplifier output circuit. The oscillator output circuit includes the secondary winding 283 of transformer 20I. The amplifier grid circuit includes a part of the volume control switch or potentiometer I43 and the grid biasing resistor 209, the latter being shunted by the condenser 2 I0. The potentiometer winding I43 is included in an output circuit of tube I which includes the condenser I96 and the resistor I91, and constitutes a variable coupling between such output circuit and the amplifier grid circuit. The oscillator grid circuit includes the tertiary winding .204 of the transformer 2M and the resistor 2I I,-shunted by condenser 2 I 2. *5

The amplifier and the various circuits thereof will be described more fully in the course of the detailed explanation of the operation .of the machine.

Direct current :for operating the relays X and IBI and the solenoids 58, '88 and 89 is supplied :by a rectifier which includes the rectifier tube 220 and filter condenser 222. .A separate rectifier. comprising the tube 22I and the voltage divider and filter condensers as shown is providedifor supplying-direct current to the tube circuits. The heaters for the'tubes (not shown) maybe'con nected in series across the alternating current supply line inparallel with the motor 29,. The conductor 236 is a common negative bus or termi nal for ,both rectifiers. The conductor 23.! is the positive bus or terminal of the rectifier which comprises tube 220. The other rectifier hasthree positive terminals .on-the voltage divider'to which the anodes andscreenigrids of the tubes are connected according "to known-practice.

The operation .of the machine when used '"as :an office dictating machine will now be explained. It may be'rassumed for this purposethatthe supply spool 3I and the take-upspool .32 are inpositionas shown in Figs. 1 and 2 and that the wire 30 has :been started on to spool 32. The indicator 225 should be set to zero. It may be assumed also that the main switch M4 has been .closed,so that the motor26 is running and current is being supplied to the rectifiers and to the heaters ,in the tubes.

Being ready to start dictating, the operator now actua-tes the key 'A, which looks in operated position. At contact A-5, key A prepares :a circuit for theslow forward solenoid '58. At contact A3, key connects the microphone I52 to the rimary winding I1-I of the input transformer I10 over a circuit which maybe traced from ground by-way of connectorcontact -I .I mircrophone I52,

connector contact Iii, conference switch contact -4, contact X--'3',-'contact A-'3, and winding I 1| tozground; -At contact AI key A connects the secondary winding I83 of the output trans former I.8:I to'the voice coil-I65 over an obvious path which need not be traced in detail. .At contact A-4, key A connects the winding 294 .ofsthe oscillator transformer ZflI to the control grid of the :tube 200, thereby closing the oscillator grid circuit .ofrthe tube, and .at contact A2, key A connects the secondary winding 203 of the oscillator transformer 20! in series with the eras coil I61 andthe biasingcoil I96. I I

The tube 200 now starts to function as ;an oscillator, due to the coupling between windings 202 and 204, which are connected in the anode The oscillator frequency is determined mainly by the inductance :of. f-windi'ne 9 a the ,wpacity 9i the tuning condenser 205 and may be about 25 kilocycles per second. The frequency should be well above the voice frequency range. The condenser 2I3 has sufficient capacity, about .01 microfarad, to afford an effective high frequency bypass around the highly inductive winding 20'! in the anode circuit. The output of the oscillator is delivered to the erase coil E61 and biasing coil I66 over the above referred to connection established by contact A--2 between these coils and the secondary winding 203.

The operator, who holds the microphone assembly I50, Fig. 5, in one hand, now actuates the switch I54 and holds it in operated position. A circuit for the slow forward solenoid 58 is thus completed which extends from the negative condoctor 236 by way of conductor 238, key contacts 18-5 and A-5, conductor 239, contact I 45-3, connector contact 3, actuated switch I54, connector contact 5, conductor240, contact X-5, conductor 24I, centrifugal switch 6'1, and winding of solenoid 58 to the positive conductor 231. The solenoid 58 energizes over the above circuit and through the medium of the previously described mechanical arrangement couples the shaft 36 carrying the take-up spool 32 to the slow speed motor shaft 28. The take-up spool 32 starts to rotate and the wire 30 begins to move past the recording head I25. The switch 66 is now open, having been operated by the slow forward mechanism, and the normally existing short circuit has been removed from the primary winding I1 I of the input transformer I10.

In moving past the recording head the wire 30 first passes the gap with which the erase coil I6'I is associated and since this coil is now being supplied with high frequency current from the oscillatorany previously existing record on the wire is wiped out.

Voice currents generated in the microphone I52 in response to dictation are transmitted to the input transformer I10 over the circuit previously described and thence to the first stage amplifier tube I75 where they are amplified in known manner. The voice'currents are further amplified at the second stage amplifier tube I80 and the output of this tube is impressed on the voice coil I65 by means of the output transformer I8I. Thus the voice currents are recorded on the wire 30 as it passes the gap with which the voice coil h is associated. During this operation the biasing coil I66, supplied with high frequency current, functions in known manner.

As previously pointed out, the grid of the automatic volume control tube I85 is coupled t the output circuit of tube in parallel with the control grid of the amplifier tube I 80 and in consequence the voice frequency voltages impressed on the grid of the latter tube are also impressed on the grid of tube I85. During dictation, therefore, the voice currents are amplified by tube I85 and alternating voltages are generated in the secondary winding I90 of transformer I88. This winding is included in the previously referred to direct current circuit which includes also the resistors I92 and I93 and the diode elements of tube I80, and accordingly a pulsating direct current is caused to flow in this circuit, producing potentials at points 242 and 243 which are negative with respect to the potential on conductor 236 to which the cathodes of tubes I80 and I85 are connected. These negative potentials are smoothed out by the filter condenser I94 and are proportional to the value of the current flow in the direct current circuit which in coming voice frequency signals.

The point 242 is connected through the resistor I95 to the control grid of tube I and the negative potential thus impressed on this grid in response to incoming signals causes the sensi'- tivity of the tube to vary inversely with the amplitude of the signals, whereby the tube tends to maintain a substantially constant output. The regulating effect is enhanced by the connection of point 243 to the grid of tube I85, whereby incoming signals cause a negative biasing voltage to be placed on the grid of this tube also. With this arrangement the'tube I 80 can be somewhat over-compensated and the drooping output characteristics of the tube which would otherwise re-' suit is corrected by the controlled output of tube I 85. The desired result is obtained by proper proportioning of the circuit constants, particu-. larly the values of the resistors I92 and I93; which function as a voltage divider. The resistor I92 may have a resistance of about 5.6 megohms and 'in this case the resistor I93 should have a resistance of about 1 megohm, whereby the biasing potential applied to the grid of tube I in response to a given input signal will be about one-seventh the biasing potential applied to the grid of tube I80.

It is important that the total resistance embodied in resistances I92 and I93 be relatively high, to give a long time constant for the discharge of condenser I94, and thus prevent the operation of the volume control circuit from producing any audible component in the output of the amplifier. The charging circuit for condenser I94, on the other hand, has a relatively very short time constant by reason of the low impedance of the cathode plate circuit of tube I85, which includes the primary winding of the transformer I88, shunted by the resistor ISI. This circuit, which largely determines the charging time of condenser I94, may have an impedance as low as 30,000 ohms, as against the resistance of approximately 7 megohms in resistors I92 and I93. The short time constant for the charging of condenser I94 insures a very quick response, to strong signals and eliminates any noticeable delay which would otherwise be manifested in the amplifier output.

The condensers I14 and I84 which may have a capacity of about micro-microfarads, are provided to bypass any high frequency currents that may be present in the input circuit of tube 7 I15 due to stray coupling with the oscillator circuits. Such high frequency currents, amplified by tube I15, might afiect the operation of the automatic volume control circuit.

The operator will probably have to stop dictating from time to time and at such times he will release the switch I54, thereby deenergizing the slow forward solenoid 58 and uncoupling the take up spool 32. This stops the movement of the wire 30 from spool 3I to spool 32 and avoids wasting the wire. The movement of the wire can be instantly started again by again actuating the switch.

In case dictation is stopped for any reason which distracts the attention of the operator, he will wish to reproduce the last part of the record before resuming dictation, to insure proper continuity of the recorded subject matter. Reproduction of a desired part of the record involves first rewinding the section of the wire on which the matter to be reproduced is recorded, then resuming-the'slow forward movement "of the :wire, and switching the circuits fromre'cordingto reproducing condition. These operations are controlled with great facility by means of the switches associated with the hand microphone.

The micro-switch I154 being in normal or released position, the operator ac'tuates the switch :I 53 and thereby completesa circuitfor the rewind solenoid 89 which can be traced from the negative Lconductor v235 by way of conductor :238, key :contacts E-5 and A--5, conductor239, contact I.453, connector contacts 3, normally closedcontact of switch I54, conductor 2.45, lower make contact of switch I53, connector contact '4, con: .ductor 246, and winding of the rewind solenoid 89 to the positive conductor 231. Upon energizing, solenoid .89 couples the shaft 41] carrying the supply spool ill to the high speed shaft-.21 of ethe motor by means of belt 19 "as previously .described. The supply spool accordingly begins-to rotate athigh speed to rewind the wire from the takeup spool 32. A branch of the above described circuit of isolenoid 89 extends from conductor 1245 -:by way of the upper make contact of switch 1153., connector contactll, conductor s241,-winding.of:relay X,and conductor 248 to the positive conductor .231. Relay X accordingly-energizes at the .same time that-solenoid 89 is energizedand-closes :a locking circuit for itself at contactX-li.v This locking circuit includes connector contact 6 and Conductor 249.

In addition to the foregoing, relay JX prepares :a'circuit for the slow forward solenoid 58 at contact X-5 and at other contacts performs .switching operations whereby the machine is placed in condition for reproducing the record as soon as the slow forwardmotion of the wire is resumed. At contact XI the voice coil I65 is disconnected from thesecondary windingfll83 cf the output transformer I8I :andis connected .to the primary winding I'II of the input transformer IIll. At contact X3 the :microphone I52 is disconnected from the primary winding -IH of .the'input transformer I70 and isc onnected to the secondary winding 208 of the amplifier .output transformer 286. ,At contact X'4 the control grid of tube 200 is disconnected from winding 284 of the oscillator transformer I and is connected to the output circuit .of tube I80 at the volume control switch I43. This latter operation converts tube 20!! from anioscillatorzto can amplifier. Although the generation of high frequency current ceases, the primary winding .282 remains in the anode circuit of tube 20Il.and it is necessary therefore 'to open the oscillator @utput circuit, which is accomplished at contact operator will ordinarily holdct-he switch I53 in operated position for only agfew :seconds, since the high speed rewinding operation very :quickly-rewinds enough wire so thatjjby reproduc :ing the speech recorded'thereon the .operator will .be able to recall where he left off and resume .di-cta-tion. When the switch IE3 is released, the initial energizing circuit .of relay X is broken, {but the relay remains energized over :its locking "circuit. The release of the switch Ijopens'the circult of the rewind solenoid :89'and prepares a circuit for the slow forward solenoid 58, the latter circuit extendin from the negative conductor .236 by way of the previously tracedpath to conductor 245 and thence by way of the normally closed switch contact, connector contact 11, con- .ductgr 12.58, contact X-5,.conduo,ton2l centrifu sal z witch'jfil, and "winding-of isoleno sa wuie 14" positive conductor .2 3.1. The rewind solenoid :89 deenergizes when its circuit is broken, the :supply spool 3| is uncoupled from the motor, .and the spools are quickly brought torest by the braking mechanism in the manner previously described.

During the rewinding operation thecentrifugal switch opens'the circuit of the slowforward'solenoid 58, the .rewinding speed being many times faster than the slow forward speed. As the-spools are slowed down by the braking mechanism after the deenergization of solenoid 89 the centrifugal switch closes shortly before the speed becomes equal to the slow forward speed, thereby :completing the prepared circuit forzthe slow forward solenoid 58. Upon energizing, solenoid 58 couples the take-iup spool to the motor and the slow forwarddrive .is resumed. By this time thespools will have been brought to rest by the braking mechanism, or'very nearly so. The delay .in the closure of the circuit for the slow forward solenoid which is introduced by the centrifugalrswitch eliminates the danger of breakingthe wire which could be present if the slow forward drive were to be started immediately after stopping the high speed rewind drive.

The wire 36 is now moved past the recording head atslow forward speed asbefore. With the circuitsin their present condition,Ihowever,1relay X energized, the machine functions as areproducer rather than as a recorder. As thewire passes the .air gap associated with the voice coil I65, voice currents are generated in the'coil and are tr.ansmitted 'tothe input transformer 111:8 of the amplifier. Thepvoice currents are amplified at thefirst and second stage amplifiertubes I15 :and :88 in the usual manner, .and are further amplified at tube 200, now functioning as .a third amplifier stage. 'The secondary windin of the outputttransformer :286 being now connected to 'the microphone 152 the amplified Voice currents operate the microphone as a receiver to :repro- 'duce thespeech recorded on the'wire.

.It .is to be :noted that the automatic .volume control circuit associated with the second stage amplifier :tube 18.6 is in :operation during the reproducing operation as "well "as during the recording operation. In a manner of speaking, therefore, this circuit arrangement "gets a second shot at any irregularities that :there may have been in the volume "of the speech that was .recorded and is able to maintain the volume of the reproduced speech at a remarkable :constantilevel.

The value of :this constant level depends on the setting of the volume control switch I43, which can zbeadjustedtogive any desiredoutput volume.

The operator listens to the reproduction o'f'his dictation andwhen the end is reached ;he actuatesjet-he micro-switch I154 again and can immediately resume his dictation. The operation of the switch breaks the locking circuit of relay X andalso breaks the circuit of the .slow forward :solenoid 5.8. The relay Xaccordingly deenergizes and recon-verits the machine to a recorder. The slow .forward solenoid .deenergizes :momentar'ily but :its ioriginal' energizing circuit is immediately completed at the makecontact-ofthe switch, so that the :slow forward drive is continued-with no appreciable interruption. If the operator desires to merely stop the machine after the recorded matter has been reproduced he will actuate the switch I54 only momentarily, "which will unlock the 'relay X and open the circuit of the solenoid 58 as above described. The circuit of thezsolenoid 8fis not again closed upon theWrelcaseIof :the.

switohibecause relayix, upenneeneigizmg cpens the circuit at contact X5. This leaves the machine ready to start recording again whenever the operator actuates the switch I54.

In case the operator should make a mistake in his dictation and wish to correct the same, he will immediately release the switch I54 to stop the slow forward drive and will then actuate the switch I53 to rewind a short section of the wire, enough to insure rewinding that part of the wire which contains the record to be changed. Upon the release of the switch I53, the machine starts in the slow forward direction again and reproduces the recorded speech as previously explained. Listening to the reproduction, the operator recalls what he said and notes the approach of the phrase or sentence to be corrected. He then actuates the switch I54, whereupon the machine is changed back to a recording machine, and as the slow forward movement of the wire continues the record thereon is erased. The operator then takes up the dictation at the point where the reproduction left off, repeating the last few words if desired, although this is not necessary.

As mentioned hereinbefore, it may be assumed that a full spool carries enough wire for one hour or sixty minutes of dictation. The indicator drum 225 is calibrated accordingly, and the operator may, if he desires, read the indicator before starting each letter and note the readings on a slip of paper or card opposite brief titles, such as Smith, Jones Co., etc. which identify the letters.

The operator will also observe the indicator from time to time so as to note the approaching exhaustion of the wire on the supply spool 3| andwhen the indicator shows that sixty minutes dictating time has elapsed he will stop the machine. The wire may be long enough to allow some overrun, so that it will not be necessary to stop in the middle of a paragraph, or letter of reasonable length.

The wire remaining on the supply spool may now be transferred to the take-up spool by means of the slow forward drive. This is rather slow, however, and will waste time, especially if the dictation has stopped short of the full sixty minutes for which the wire provides. The operator will prefer to use the fast forward drive, therefore, for this purpose, and will put aside the hand microphone assembly and operate the key D. The operation of this key releases key A and closes an obvious circuit for the fast forward solenoid 88. Upon energizing, solenoid 88 couples the shaft 36 carrying the take-up spool 32 to the high speed motor shaft 21, by means of belt 18, and the spool 32 is rotated at high speed to quickly complete the transfer of the wire from spool 31. The end of the wire, as it arrives at spool 32, is secured by the leader in the manner previously explained.

The operator now stops the fast forward drive by releasing key D, completes the winding of the leader on spool 32 if necessary, and removes the spool from the machine. This spool may now be passed along to the transcribing operator, with the slip of paper or card on which the location of the various letters was noted.

- jIfhe-empty spool 3I is now transferred from hub 39 on shaft 40 to hub 35 on shaft 36 and telephone conversation n which he is engaged with some outside party. The machine is adapted for this service, as will now be explained briefly.

It will be understood that a tap to the operators telephone line has to be provided, terminating in a plug which may be inserted in the jack I41. This plug may be left in the jack. The main switch I44 should be closed also, whenever it is possible to anticipate the need for the service, as otherwise the necessity for warming up the tubes may cause part of a conversation to be lost.

Assuming that a telephone conversation is about to begin and that the operator desires to make a record of it, he will actuate the key E. The key locks in operated position and at contact E+-5 completes an obvious circuit for the slow forward solenoid 58. Upon energizing, the solenoid 58 starts the slow forward drive in the manner previously explained. At contact E3 the key connects the primary winding I1I of the input transformer I18 to the telephone line by way of jack I41. At contacts EI, E'-2, and E4 the key completes other circuits by means of which the machine is placed in condition for recording. These circuits are the same as previously described circuits which are completed by key A'when the machine is in use for recordin dictation and need not be described again. The operation of recording is also the same, except that the voice currents to be recorded now come from the telephone line via jack I41 instead of from the microphone I52.

When the telephone conversation is over with the operator will stop the machine by operating the release key R. to release the key E, or by operating key A which releases key E and at the same time places the machine in condition for recording dictation. The operator can immediately reproduce the conversation if he desires to do so, by operating the switch I53 to rewind the wire and then releasing the switch to start the reproduction, all as previously explained. Later on the recorded conversation may be transcribed along with the other recorded material on the wire.

The machine is also adapted for use in recording a conference, and for this purpose a dynamic type of loud speaker is preferably provided adapted' to pick up conversation from any part of the conference room and to reproduce the recorded speech with sufilcient volume so that it can be heard all over the room. It will be understood that the hand microphone I52 is not well adapted for this purpose. The hand microphone assembly including the switches I53 and I54 is not used, therefore, although if the conference is held in the office where the machine is installed the plug I48 may beleft in the socket I48 without interfering with the operation of the machine.

The loud speaker is connected with the machine by means of a plug inserted in the jack I46. If not already done a full supply spool should also be placed in position and started on to the take-up spool. When the people are assembled and the conference is about to begin, the main switch I44 is actuated, also the conference switch I45 and the key A. At contact A-5 key A closes a circuit for the slow forward solenoid 58 which extends from the negative conductor 236 by way of conductor 238, key contacts E-S and A-5, conductor 239, conference switch contact I 453, conductor 24B, contact X-5, conductor 2, centrifugal switch 61, and winding of the slow forwaidisolenoid 58 to the positive conductor 231. lihe slow forward [solenoid accordingly energizes and starts the slow forward drive. At contact A3, the key A connects the loud speaker with the primary winding of the input transformer I'III over a path which includes the jack I46, the conference switch contact I45 I, contact X-3, and contact A--3. At other contacts key A completes the conditioning of the machine for recording, all as previously explained.

The machine may be allowed to run continuously during the conference and whatever discussion takes place will be picked up by the loud speaker and recorded on the wire. At some point in the discussion a question may arise as to .what was previously said by some one of the people present, making it desirable to reproduce the record. This is accomplished by first operating key C to rewind the wire, only enough need be rewound to include the statement to be verified, and by then operating key B to reproduce the record on the wire that has been rewound.

Explaining these operations more in detail, the actuation of key releases key A and the releases of this key deenergizes solenoid 58 and stops the slow forward drive. Key C also closes a circuit for the rewind solenoid 89, which functions as previously described to start the rewind drive. After suificient wire has been rewound, key B is operated, releasing key C and stopping the rewind drive.

Key B closes a circuit for the slow forward solenoid which may be traced from the negative conductor 236 by way of conductor 238, key contacts E5, A5, and B5, conductor 252, conference switch contact I452, conductor 240, contact X5, conductor 24 I, switch 61, and winding of solenoid 58 to the positive conductor 231. Upon energizing, solenoid 58 starts the slow forward drive in the usual manner. At contact B3, key B connects the voice coil I65 to the primary winding I'II of the input transformer I10.

At contact B I the control grid of tube 209 is connected to the output circuit of tube I89, and at contact B2 the short circuit is placed on the secondary winding 203 of the oscillator transformer 20L Finally, at contact BI, the secondary winding 208 of the output transformer 206 is connected to the loud speaker, this latter connection including the conference switch contact I45I. These switching operations cause the machine to function as a reproducing machine, utilizing the loud speaker to repeat the previous discussion, or so much of it as is recorded on the section of wire that was rewound, to the members of the conference. When the reproduction is finished, key A is operated, releasing key B, and converting the machine back to a ecording machine again, ready to record the comments on the previous discussion and any further discussion that may ensue.

At the start of the reproduction of the conference discussion as above described the volume control switch I43 is adjusted so that the volume is sufficient to enable the reproduction to be heard in all parts of the room. The automatic volume control is in operation both during recording and reproducing and is particularly useful and effective in this case, that is, when the ma chine is employed for conference service. Words spoken directly into the loud speaker and words spoken across the room, or twenty or thirty feet away, are reproduced with substantially equal volume. There is, moreover, an almost instantaneous' response, which makes it possible for persons close..to and far away from the loud 1'8 speaker to talk in quick succession without impairing the uniform level of the reproduced conversation.

When the conference is over the wire remaining on the supply spool is quickly wound on to the take-up spool by utilization of the fast forward key D and the latter spool is taken off the machine and handed over to the transcribing operator or typist for preparation of a typewritten transcription.

The foregoing completes the explanation of the dictating and analogous recording operations. The operations performed by the transcribing operator will now be explained.

The machine used by the transcribing operator may be the same as the machine used for dictation, except that the hand microphone assembly I is not supplied but is replaced by the head phone I51 and the foot switches I58 and I 59. It may be assumed therefore that the identical machine which has been described hereinbefore is now a transcribing machine and that the plug I60 in which the cords from the head phone and the foot switch assembly are terminated is inserted in the socket I48.

When a spool of record bearing wire is received by the transcribing operator she will place the same on hub 35, where it retains its identity as the take-up spool. An empty spool is then placed on the hub 39 and becomes the supply spool. The end of the wire is then started on to the supply spool by means of the attached leader, as previously explained, and the wire between the spools is properly adjusted on the recording head. Assuming that the main switch I44 is closed, the operator will note the position of the indicator drum 225 and set it to zero if it is not in that position already, and will then operate the rewind key C, which looks in operated position. Key C closes the circuit of the rewind solenoid 89, which starts the rewind drive whereby the supply spool is rotated at high speed to transfer the wire from the take-up spool. During this operation the operator will observe the indicator drum 225 and will release key C to stop the rewinding operation as soon as the pointer again reads zero on the scale, the drum having made one complete rotation. This leaves the wire attached to the takeup spool, although the entire record bearing por-v tion of it has been transferred to the supply spool.

Having adjusted the head phone l5! if necessary, and seen to it that the foot switch assembly is in convenient position, the operator will actuate the key B, which locks in operated position and places the machine under control of the foot switches. Key B also closes various circuits which condition the machine for operation as a reproducing machine.

The operator may now momentarily depress the switch I58 which momentarily closes a circuit for relay I 6 I said circuit extending from the negative conductor 236 by way of conductor 238, key contacts E5, A-5, and B5, conductor 252, conference switch I452, connector contact 8, normally closed contact of switch I59, switch I58, connector contact I, conductor 253, and winding of relay I6I to the positive conductor 231. Upon energizing, relay I6I advances the ratchet wheel I62 one step and on deenergizing prepares to advance it another step when again energized. The cam IE3 rotates with ratchet wheel I62 and at contact I64 closes a circuit for the slow forward solenoid 58. which may be traced as a branch of she previouslytraced circuit from switch I58 by way of connector contact 2, conductor 254, contact I64, centrifugal switch 61, and winding of the solenoid 58 to the positive conductor 23?. Upon energizing, the solenoid 58 starts the slow forward drive in the usual manner. It will be noted that contact I64 remains closed and the machine continues to run independently of the foot switch I58 which was only momentarily operated.

The key B being in operated position the machine 'now operates as a reproducing machine and the speech recorded on the wire is reproduced in the head phone I51, where it is heard by the operator. In this connection it will be noted that the head phone has been substituted for the microphone I52 and is connected to the secondary winding 208 of the output transformer 206 over a circuit previously traced.

The operator writes the letter or other recorded material on her typewriter as it comes in at her head phone. If she should get behind she can stop the machine by momentarily operating the foot switch I58 again, which operates the relay IBI again and advances the ratchet wheel I62 and cam I63 another step. This opens contact I64 and deenergizes the solenoid 58. When the operator has caught up with the machine, she can start it again by another momentary operation of the foot switch I58.

In the event that the operators attention should be diverted, causing her to forget some phrase before it can be written out, or if she should be uncertain of some passage, she will want to repeat the reproduction of such phrase or passage. This is accomplished by depressing the foot switch I59.

The operation of switch I59 breaks the circuit of solenoid 58 if this circuit is closed at contact I64 at the time the switch is operated. This stops the slow forward drive. Switch I59 also closes a circuit for the rewind solenoid 89 extending from the negative conductor 236- by way of conductor 238, key contacts E-S, A-5, and 3-5, conductor 252, connector contact 8, switch I59, connector contact 4, conductor 246, and winding of solenoid 89 to the positive conductor 23?. Upon energizing solenoid 89 starts the rewind drive in the usual manner. The operator holds the switch I59 depressed only for an instant, or long enough to rewind that part of the wire which contains the record of the words which she desires to have repeated, and then releases it. The release of the switch deenergizes the rewind solenoid 89 and stops the rewind drive, also again closes the circuit of the slow forward solenoid 58, if. the circuit was closed at the time the switch wasoperated. In this event the solenoid 58 is immediately energized again and the slow forward drive is resumed. Otherwise the slow forward drive is started by momentary operation of foot switch I58 when the operator is ready to proceed.

It will be seen that the foot switch arrangement enables the operator to manipulate the wire with great facility. The forward drive and reproduction of the record can be started and stopped as desired, by momentary operations of switch I58. The switch does not have to be held down, which enables the operator to give her undivided attention to typing the recorded material as it is reproduced. The arrangement of switch I59, which requires it to be held operated during rewinding, is likewise welladapted to the function it has to perform. During rewinding the opera tors attention is occupied solely with this opera tion and since the operation is stopped by simply releasing the switch she is able to accurately control the duration of the operation in accordance with the amount of wire to be rewound each time.

Ordinarily the operator will transcribe the material recorded on the wire in the order in which it was dictated. The fact that the wire may contain a full hours dictation, however, may make it necessary for her to depart from this practice and transcribe the several parts of the record in a different order. As an extreme case, the last letter recorded on the wire may be wanted first. This could happen, for example, when a spool is filled toward the end of the day when there is not sufficient time to transcribe the entire spool of wire, and so far as most of the ma terial is concerned there is no need to do it, but it is necessary that the letter last dictated be transcribed and mailed that day. I

In such a case the operator will make use of the fast forward drive to quickly bring the desired letter into position for reproduction. After noting the location of the desired letter on the wire, as shown by the indicator slip accompanying the spool, the operator will operate the key D which locks and closes the circuit of the fast forward solenoid 88. Solenoid 88 energizes and initiates the fast forward drive. The operator now watches the indicator drum 225 and stops the fast forward drive by releasing key D when the pointer 23I which is associated with the scale on the drum indicates that the record of the desired letter has been reached. Key D may be released by operating key B, which places the machine in condition for reproduction. The exact beginning of the letter can then be -located readily by starting the slow forward drive and listening to the reproduction, rewinding some of the wire if necessary. a

In connection with the foregoing it may be pointed out that if contact I64 on relay IBI is closed at the time key B is operated the energization of the slow forward solenoid 58 will nevertheless be delayed by the centrifugal switch 67 until the rotational speed of the spools has slowed down nearly to the slowforward speed. This prevents the abrupt reduction in the speed of the take-up spool and overrun by the supply spool which would otherwise occur.

It will be noted that during the rewind and fast forward drives the control grid of tube 200 is connected to the negative conductor 236 by key contact C#. or 13-4, as the case may be. This may not be strictly necessary but is believed to be desirable in order to prevent incipient oscillations which might damage the record because of capacitive coupling between the oscillato circuit conductors and conductors extending to the Voice and erase coils.

The invention having been described, that which is believed to be new, and for which theprotection of Letters Patent is desired will be pointed out in the appended claims.

We claim:

1. In a sound recording and reproducing machine, a wire or other paramagnetic medium on which sound is recorded, a voice coil and an erase coil associated with said medium, a multistage amplifier for amplifying currents generated in said voice coil when said machine is used as a reproducer and for transmitting amplified sound currents to said voice coil when said machine'is used as a recorder, and switching means effective when said machine 'is' used-as.

21 a; recorder for cutting out the last amplifier stage. and for converting the same to an oscillator for supplying high frequency currents to said. erase coil.

2. In a sound recording and reproducing machine, a wire or other paramagnetic medium onwhich sound is'recorded, a voice coil and an erase coil associated with said medium, an amplifier having at least two serially related output stages, a sound power telephone, switching means effective when said machine is used for reproducing for connecting said voice coil to the input of said amplifier and for connecting the second output stage of said amplifier to said telephone, switching means effective when said machine is used for recording for connecting said telephone to the input of said amplifier and for connecting the first output stage of said amplifier to said voice coil, and additional switching means effective during recording for converting said second output stage to an oscillator for supplying high frequency current to said erase coil.

3. In a sound recording and reproducing machine, a wire or other paramagnetic medium on which sound is recorded, a voice coil and an erase-coil associated with said medium, a multistage amplifier, switching and circuit means for utilizing all stages of said amplifier to amplify currents generated in said voice coil, and switching and circuit means for utilizing all the stages 'otsaid amplifier except the last stage for amplifying and transmitting sound currents to said voice coil, for operating said last stageas an oscillator, and for transmitting the oscillator output to said erase coil.

. 4. In an amplifier for a sound recording and reproducing machine having a voice coil for cooperation with a record medium, said amplifier having at least two stages, a combination amplifier and oscillator tube in thefinal stage .cr. said amplifier, amplifier and oscillator output circuits for said tube, an input circuit for said final stage extending from the amplifier stage preceding said final stage, a feed back circuit coupled to the oscillator output circuit, and switching means for connecting the control grid of said tube either in said input circuit or in said feed back circuit, said switching means including means for completing an output circuit extending from said preceding stage to said voice coil at times when said'control gridv is connected in said feed back circuit. 5. In an amplifier for a sound recording and reproducing machine, a combination amplifier and cscillator'tube, two grid circuits for said tube,

means for closing either or said grid circuitsto ppferate said tube as an amplifier or as an oscillator, an anode circuit for said tu be and amplifier output and oscillator output transformers having their primary windings included. in said anodecircuit in series.

6; In an amplifier as claimed in claim 5, a tuning condenser connected in shunt of the primary winding of the oscillator transformer and a bypass condenser connected in shunt of the primary output current, connections extending from the cathodes of said tubes to a point on said direct current circuit, and connections extending from the grids of said tubes to points on said direct current circuit which are negative with respect to said first mentioned point when current is flowing in said direct current circuit.v

.8. In a sound recording and. reproducing machine, a microphone, a voice coil, an amplifier; means for connecting said amplifier to receive signal currents from said microphone or said voice coil at will, an amplifying tube in said amplifier responsive to said signal currents, automatic volume control means including a second amplifying tube, means for diverting a. desired fraction of said signal currents to said second tube, a direct current circuit to which power is supplied by said second tube in response to said signal currents, and means for utilizing potentials. on said direct current circuit to control the sensitivity of said tubes.

9. In a sound recording and reproducing machine, a microphone, a voice coil, an amplifier, means for connecting said amplifier to receive signal currents from said microphone. or said voice coil at will, an amplifying tube in said amplifier responsive to said signal currents, automatic volume control means including a second amplifying tube, means for diverting a desired fraction of said signal currents to said second tube, a direct current circuit to which power is supplied by said second tube in response to said signal currents, means for utilizing a negative potential derived from said direct current circuit to place a variable bias on the grid of said first tube, and means for utilizing a negative potential derived from said direct current circuit to place a variable bias on said second tube to prevent over-biasing of said first tube in response to high amplitude signal currents.

10. In a sound recording and reproducing machine, a record medium, a voice coil cooperating with said medium, an input circuit, an amplifier, means for connecting said amplifier to receive signal currents from said, input circuit or said voice coil at will, an amplifying tube in said amplifier responsive to said signal currents, automatic volume control means including a second amplifying tube, means for diverting a fraction of said signal currents to said second tube, means including a rectifier for deriving two potentials from the output of said second tube which are continuously negative with respect to the oathodes of said tubes, and means for applying said two pot'entials to the grids of said two tubes, respectively. I

11. In a sound recording and reproducing machine, an audio frequency input circuit, a voice can, an amplifier, means for connecting said amplifier to receive signal currents from said input circuit or said voice coil at will, an amplifying tube in said amplifier responsive to said signal currents, automatic volume control means including asecond amplifying tube, means for divertinga desired fraction of said signal currents 'to said second tube'jfmeans for rectifying the output of said second tube to produce a con-'- tinuous voltage which is negative with respect to the cathode of said first tube, and means for applying said voltage to the grid of said first tube to regulate the output thereof.

12. In a sound recording and reproducing machine, a voice coil adapted for cooperation with a record medium, a stage amplifier, an output circuit leadingfroni the secondv stage of said 23 amplifier to said voice coil for use during recording, a coupling circuit leading from the second stage of said amplifier to the third stage thereof for use during reproducing, and switching means for selectively connecting said circuits.

13. A sound. recording and reproducing machine as claimed in claim 12, wherein means is provided for operating the third stage of said amplifier as an oscillator during recording, together with a circuit over which the output of said oscillator is supplied to an erase coil associated with the record medium.

14. In a sound recording and reproducing machine, a three stage amplifier, a voice coil, an output circuit leading from said second stage to said voice coil for use during recording, a coupling circuit leading from said second stage to said third stage for use during reproducing, a sound current translating device, an output circuit leading from said third stage to said device for use during reproducing, means for selectively connecting said circuits, an erase coil, and means operative during recording for operating said third stage as an oscillator to supply current to said erase coil.

15. In a sound recording and reproducing machine, a voice coil, an erase coil, a two stage amplifier operative to amplify incoming speech currents and transmit the same to said voice coil during operation of said machine as a recorder, an oscillator for supplying current to said erase coil, a sound translating device, means for converting said oscillator to a third amplifier stage, and means for connecting the three stage amplifier thus formed between said voice coil and said translating device during operation of said machine as a reproducer.

16. In a sound recording and reproducing machine, a wire or other paramagnetic medium on which sound is recorded, a voice coil and an erase coil associated with said medium, means including an amplifier for operating said voice coil during use of said machine as a recorder, an oscillator for supplying current to said erase coil, and means for converting said machin to a reproducer, including a sound current translating device, means for connecting said voice coil to the input of said amplifier, means for converting said oscillator to an amplifier, and means for operatively connecting said last mentioned amplifier between the output of said first mentioned amplifier and said translating device.

17. In a sound recording and reproducing machine, a wire or other paramagnetic medium on which sound is recorded, a voice coil and an erase coil associated with said medium, an amplifier for amplifying incoming audio frequency currents for transmission to saidvoice coil for recording, an oscillator for supplying current to said erase coil, and switching means for converting said machine to a reproducer, said means including means for connecting said voice coil to the input of said amplifier, means for coupling the output of said amplifier to said oscillator and for operating the latter as an amplifier stage, a sound current translating device, and means for connecting the sam to the output of said amplifier stage.

18. In a sound recording and reproducing machine, electromagnetic means for recording and reproducing audio frequency currents, an amplifier, manually operated switching mechanism, circuits completed by said mechanism for operating said machine as a recorder with said amplifier operating to amplify the currents to be recorded, circuits adapted to be completed by said mechanism for operating said machine to reproduce the currents so recorded, with said amplifier operating to amplify the reproduced currents, automatic volume control means, and means includins. said circuits for operating said volume control means to regulate the output of said amplifier both when the machine is operating as a reproducer and as a recorder, whereby variations in the amplitude of the reproduced audio frequency currents resulting from imperfect regulation of the amplifier during the recording operation are substantially eliminated.

19. A sound recording and reproducing machine as claimed in claim 18, wherein the number of amplifying stages in said amplifier differs depending on whether the machine is operating as a recorder or a reproducer and wherein the automatic volume control is applied at a stage which is in use both when the machine is operating as a recorder'and as a reproducer.

20. In a sound recording and reproducing machine, a combination two and three stage amplifier having an input circuit and second and third stage output circuits, a recording and reproducing head including a voice coil, a translating de-'- vice operative both as a voice current transmitter and receiver, means for operating said machine as a recorder with said translating device connect'ed to saidinput circuit and said voice coil connected to said second stage output circuit, means for operating said machine as a reproducer with said voice coil connected to said input cir cuit and said translating device connected to said third stage output circuit, and means comprising electrical conductors and switching mechanism for operating said machine and connecting said elements as set forth.

ALLEN A. SHOUP. RUDOLPH A. BIERWIRTH. EARL L. OLSON.

REFERENCES CITED The following references are of record in the file of this patent:

OTHER REFERENCES Journal of the Society of Motion Picture Engineers, vol. 48, June 1947, pages 560 to 568, A

High-Quality Recording Power Amplifier, Kurt-Singer. 

